r′-band magnitude distribution for objects in XXL-N (top panels) and XXL-S (bottom panels). Left column: histograms for counterparts: grey refers to all candidate counterparts (primaries and secondaries); black to the primary counterparts; and green to the subset of the primaries with a spectroscopic redshift. Right column: densities n(m) for all objects in specific surveys. For CFHT this amounts to more than 5 million objects in ~43 deg²; for BCS (bottom panel, black solid curve) to approximately 2 million objects in ~47 deg²; and for the deeper DECam survey (dashed blue curve) to approximately 2 million in just ~16 deg².